This application claims foreign priority benefits under 35U.S.C. xc2xa7119 from United Kingdom Patent Application Serial No. 9818254.6 filed Aug. 21, 1998, and under 35 U.S.C. xc2xa7120 from PCT Patent Application Serial No. PCT/GB99/02736.
The present invention relates to an apparatus for determining the inflation pressure of a pneumatic tire mounted on the wheel of a vehicle. It also relates to a method of measuring such pressure.
Almost all motor vehicles in use today utilize pneumatic tires, that is to say, tires that depend on an internal gas pressure for their proper functioning and reliability. Vehicle and tire manufacturers go to considerable lengths in the design of their products to achieve optimum performance, and they consequently carefully specify the match between the tire type and the vehicle and the operating conditions of the tire. Hence pneumatic tires should be maintained at appropriate pressures in order to achieve optimum safety, comfort, road handling, fuel economy and tire longevity.
In ordinary use, pressures in tires vary considerably, and this can be due merely to slow leakage, for example due to permeation over time or temperature variation, or can be due to faster leakage instead. The latter can be due to faults or episodic pressure loss, for instance because of impact when riding over a curb. For all these reasons, frequent checks on the inflation pressure of pneumatic, tires are recommended by all authorities.
The ordinary process of checking tire pressure, however, is generally user unfriendly, since it firstly requires the removal of the dust cap, and occasionally other parts of the wheel trim, all of which are usually covered in road dirt, and secondly requires the awkward manipulation of a tire pressure gauge. The unattractive nature of these tasks often discourages regular checking, with consequent effects on fuel economy, tire longevity and vehicular safety.
European Patent Application No. 0545641A discloses a system for determining the pressure of pneumatic vehicular tires, by using a linear array of load sensors arranged to provide a pattern of force distribution data exerted by each tire in contact with the sensors, measurements being taken along a line across the width of each tire. A computer is used to determine the tire pressure from the force distribution data, with the tire pressure being determined by a decomposition method or an analysis technique, such as determining the pseudo pressure, and involves the use of neural networks.
European Patent Application No. 0656269A describes a system similar to that disclosed in European Patent Application 0545641A, but provides a two dimensional array of load sensors giving a two-dimensional (rather than one dimensional) pattern of the force distribution exerted by the tire. This is achieved by either a linear array of sensors in contact with the tire and along a line across the width of the tire as the tire rolls thereover, or by a two- dimensional array of sensors in contact with the tire. Again, the analysis technique used involves the extracted data being fed into a neural network.
Such prior systems involve complex processing of much data, and an aim of the present invention is to provide an apparatus and method which are able to determine inflation pressure quickly, easily and in a much simpler way, primarily to a first order of accuracy. The present invention also aims to allow such a first order of accuracy measurement to be further refined to give a more accurate determination of inflation pressure.
Thus, according to a first aspect of the present invention, an apparatus is provided for determining inflation pressure of a pneumatic tire mounted on the wheel of a vehicle, the tire being in contact with a support over a contact area, said apparatus comprising:
(i) sensing means comprising one or more sensors within said support, each sensor adapted to measure the average contact pressure over the sensor between said tire and said support, and each having a surface area less than the said contact area; and
(ii) processing means adapted to determine an average contact pressure between said tire and said support from output signals of only a sensor or sensors which fall entirely within said contact area, so as to provide a measure of the inflation pressure of the tire to a first order of accuracy.
If a tire is considered hypothetically to be composed of material forming a perfect membrane, such a tire would have a circular profile when inflated, but unloaded. However, when such a perfect tire is loaded, the area of the tire in contact with its support (for example the road) is typically substantially flat. Increases in total load on such a tire to a first degree of order merely result in a greater contact area, that is area of tire in contact with the support.
However, in practice, tires cannot be assumed to be perfect, but instead are subject to factors such as tire construction, stiffness and tread type which affect the fringe of the contact area. If such fringe effects are ignored, the tire behaves more like a perfect membrane and thus by ignoring fringe effects it can be expected that increases in total load merely result in a greater contact area.
By ignoring fringe effects, although the total load on a tire is more closely related to contact area, it cannot be said that the relationship between the two parameters is necessarily linear, and a purely one to one relationship is departed from.
In practice, the relationship can be more precisely defined by a calibration curve, an algorithm or database of standard values of inflation pressure for known values of contact pressure, by testing to see what contact pressure arises with a given inflation pressure and repeating with several different inflation pressures. Once the calibration curve, algorithm or database has been defined by repeated tests, it can be used to determine inflation pressure from given readings of contact area that exclude fringe effects.
Typically, tire contact pressure (that is, the external contact pressure between a tire and its support) is sensed, in one embodiment of the invention, over one or more of a series (or a cluster) of small sensors as the vehicle travels over them. The size of these sensors is made sufficiently small so that the tire contact area entirely covers at least one of the sensors. Outputs from sensors which are only partially covered by the tire, are excluded. However, the sensors must also be sufficiently large, or numerous, to average out local contact pressure variations, for example due to tread pattern.
The specific dimensions of the individual sensors are chosen such that more than one sensor is completely covered by the contact area between a tire and its support. Typically, the signal processing of the outputs from said sensors is arranged so that fringe effects, represented by the outputs of those sensors that are only partially covered by the tire, are ignored. Thus, a better estimate of the actual tire pressure is determined than would be the case if only one sensor was used, or if the outputs of all sensors whether wholly or partially in contact with the tire were used.
The clusters of sensors serve to sample average contact pressure over the contact area between a tire and the support while ignoring readings near the edges of the contact area, and thus provide a more reliable reading of the average pressure between support and tire.
In practice, the apparatus comprises clusters of sensors located to read tires on both lateral sides of a vehicle. Typically the outputs from the clusters on both sides are read by processing means at the same time, unless the vehicle is for example a motorcycle.
Typically, the outputs of the relevant sensors are read repeatedly at short time intervals by the processing means, for example, at one hundredth of a second intervals.
A significant feature of the present invention is that the system does not involve the measurement of the actual area itself over which the tire makes contact with the road, but instead involves a measurement of the contact pressure since, in this simples form of the invention, the actual area and thus the total tire load is not of immediate interest.
Preferably, the sensors incorporate plates, the pressure on each plate being sensed by the use of a mechanical flexure and strain gauge. The arrangement of the clusters of sensors, and the precise method of processing of the signals outputted from each sensor, is such that the individual pressures of more than one tire on the same side of an axle may be determined simultaneously at any one time. A double wheeled arrangement is common in commercial vehicles.
When the sensors incorporate plates, the plates may be rigid and typically are circular in plan view. The plates are usually arranged so as to receive tire load in a substantially perpendicular manner.
A variety of sensors may be used and instead of the preferred use of a mechanical flexure and strain gauge, the sensor can also be manufactured from materials that include, for example, piezoelectric materials, piezoresistive materials, and pressure sensitive optical fibres. Typically, the sensing area of each sensor provides a smooth, flat surface profile without bumps, and the deflection of the sensing area of each sensor is small. By meeting such criteria, it can be ensured that the tire contact area is flat, and so the resulting pressure on the contact area is typical.
The apparatus, as described above, provides a first order accuracy estimate of inflation pressure according to the invention in its simples form, but in order further to reduce the inaccuracies caused by edge, stiffness and other peripheral effects, overall accuracy may be improved by additional processing of the output signals from the pressure sensors. To this end, in a more refined version of the invention, it is preferable that not only is the average contact pressure between tire and road calculated as described above, but also calculated are such variables as vehicle track (that is, the distance between wheels on the same axle), number of axles, and number of wheels, which are all deduced from the same sensor readings. These latter variables are then used by the processing means to select correction factors, which are used to modify the determined value of average contact pressure. Optionally, the list of variables used in this way may be further extended to include for instance the vehicle wheel base (that is, the axle separation), the total load on each wheel, and the contact area width and length (excluding peripheral readings). Values for each of these variables are calculated (see below) by the processing means by taking outputs of only those sensors falling entirely within the contact area, and not from all sensors in contact with the tire over the entire width. Thus, as in the measurement of the inflation pressure to a first order of accuracy, peripheral contact effects are avoided when calculating inflation pressure to a greater degree of accuracy in this way.
By measuring such variables, the processing means of the apparatus can build corrections into its calculation of inflation pressure by incorporating suitable algorithms or calibration curves that compare actual variables measured to standard variable values held in a reference database, for example entered from standard data tables. Typically, such correction factors can be inputted into a database which can be accessed by the processing means of the apparatus. Inputting can be achieved by performing a series of tests on the apparatus across a standard range of conditions, that is using a range of vehicles, vehicle loadings and inflation pressures, so that the apparatus can build up its database xe2x80x98memoryxe2x80x99 of standard values and can be used afterwards to provide a more accurate estimate of inflation pressures under specific vehicle conditions as required.
In a preferred embodiment, the apparatus is adapted to compensate for the effects of tire temperature variation by making use of means included in the apparatus to sense tire temperature. Such compensation is desirable since temperature variation can significantly alter inflation pressure. Temperature may be measured by conventional infra-red or other means and used as a further input to the selection of correction factors, previously built into the database of the apparatus, which are used by the processing means to calculate an accurate estimate of inflation pressure.
In another preferred embodiment, two or more clusters of sensors are provided to measure tire pressure for tires on wheels located on opposite sides of a vehicle, clusters being longitudinally off-set, making it possible to derive two or more velocity calculations separated in time. Comparison of such velocities permits better detection of any acceleration or deceleration of the vehicle, which will affect wheelbase estimates. If undue acceleration or deceleration is sensed, a repeated test run can be requested at a more nearly constant velocity, so ensuring wheelbase is estimated sufficiently accurately.
According to a further preferred variant, the individual sensors within a cluster may be distributed in a random (or quasi-random) fashion, so as to minimise the risks of errors arising from any correspondence of the tread pattern to the pattern of sensors.
According to a second aspect, the present invention provides a method of determining inflation pressure of a pneumatic tire mounted on the wheel of a vehicle, the tire being in contact with a support over a contact area, said material comprising the following steps:
(i) measuring the average contact pressure between said tire and said support over each sensor within said support, each sensor having a surface area less than said contact area; and
(ii) processing output signals from only a sensor or sensors which fall entirely within said contact area, to determine an average contact pressure and so provide a measure of the inflation pressure of the tire to a first order of accuracy.
Other optional features of present invention will be evident from the accompanying dependent claims.